How do you know what the Thrall bonus is?
I have one that gives me new weapon blueprints.
My Armorer shows nothing.
So only tier 3 and better make special armor I believe. The armor they make depends on what race they are.
What does a nordheimer priest do anything?
Nordheimer Priest is for Ymir religion. All craftsman thralls give bonus recipes. T1 give exceptional basic items, T2 give more exceptional items, T3 and T4 give some flawless items. The items depend on the race of the crafting thrall.
Also, Exile thralls (nospecific race) give no extra recipes.